Turbulent flow drag reduction

ABSTRACT

The present invention relates to apparatus for influencing fluid flow over a surface, and more particularly, but not exclusively, to turbulent boundary layer flow drag reduction for an aircraft. The present invention provides such apparatus including a plasma generator comprising a first electrode ( 7 ) and a signal generator ( 18 ), the apparatus being operable to drive the first electrode ( 7 ) with a pulsed signal generated by the signal generator ( 18 ) thereby to cause a change in direction of the flow of the fluid over the surface.

[0001] The present invention relates to apparatus for influencing fluid flow over a surface, and more particularly, but not exclusively, to turbulent boundary layer flow drag reduction for an aircraft.

[0002] The boundary layer is a thin layer of fluid (air) that forms, for example, on an aircraft wing during flight adjacent to the surface of the wing in which viscous forces exert an influence on the motion of the fluid and in which the transition between still air and the wing's velocity occurs. Boundary layer control techniques are known where the airflow in the boundary layer is modified to increase and/or decrease drag.

[0003] Turbulent boundary layer flow is not yet fully understood, but the recognition that coherent structures exist has allowed efforts to be directed to modifying and/or controlling the turbulent boundary layer flow, as described, for example, in AIAA paper 96-0001—“Control of Turbulence”—J.Lumley. Direct numerical simulation of the turbulent boundary layer flow, as described, for example in Phys. Fluids A 4 (8)—“Suppression of Turbulence in Wall-bounded Flows by High Frequency Spanwise Oscillations”—W. J.Jung, N.Mangiavacchi, R.Akhavan shows that disrupting the coherent structures could have a dramatic effect on the skin friction, reducing it by up to 40%. If this level could be achieved on an aircraft this would equate to a reduction in total drag of between 10% and 20% offering substantial savings in fuel and/or increases in range. Experimental verification of this numerical prediction has been achieved and published in AIM paper 97-1795—“Turbulent Boundary Layer Control by means of Spanwise-wall Oscillation”—K-S.Choi, P. E.Roach, J-R. DeBisschop, and B. R.Clayton. In that paper the use of mechanical oscillation is described to demonstrate skin friction reductions of up to 45%. However, the paper does not suggest how a practical mechanical oscillation system could be implemented successfully.

[0004] Other approaches for actively disrupting the coherent structures, such as blowing, or using tiny micro-electro-mechanical actuators have been postulated, but no practical and effective means have been demonstrated. Passive modification of the coherent flow structures has also been attempted, for example, riblets and large eddy break-up devices. These approaches have achieved skin friction drag reductions, but at a much smaller level (less than 10% as opposed to 40%) and are therefore marginal in their overall benefits once extra cost and other penalties (such as increased weight) are considered.

[0005] There remains a need for a passive or active system that disrupts the coherent turbulent boundary layer structures to achieve large skin friction reductions, which is both practical and cost effective. In the 1998 conference publication AIAA 36th Aerospaces Meeting, paper AIAA 98-0328—“Boundary Layer Control with a One Atmosphere Uniform Glow Discharge Surface Plasma”—Reece-Roth, Sherman and Wilkinson, an electrode system based on rigid printed circuit board material is described and the interaction of surface plasmas with boundary layers related. The electrode system comprises a single set of a multiplicity of parallel conductive lines all electrically connected to one another. The plasma generating circuit is a high voltage radio frequency source operated at 3.0 kHz. The interaction of the surface plasmas with the airflow was said in this paper to be due to an electrostatic attractive force—termed paraelectric. At the end of the paper it is postulated that this sort of technology might be applicable to the generation of span-wise oscillations for turbulent drag reduction. No information on how this concept could be achieved was given.

[0006] Both travelling wave and span-wise oscillation boundary layer disturbances have been tried for drag reduction in sea water (using a combination of magnetic and electric field forces). Travelling waves have been found to be more effective, at least under certain conditions, as described in Science 288, 1230 (2000)—“Suppressing Wall Turbulence by Means of a Transverse Traveling Wave”, Du and Karniadakis.

[0007] It is an object of the present invention to provide an arrangement for influencing fluid flow so that an object's drag can be reduced.

[0008] According to a first aspect of the present invention, there is provided apparatus for influencing fluid flow over a surface, the apparatus including a plasma generator comprising a first electrode and a signal generator, the apparatus being operable to drive the first electrode with a pulsed signal generated by the signal generator thereby to cause a change in direction of the flow of the fluid over the surface. By providing a pulsed signal, plasma is generated in a number of discrete steps such that fluid is caused to flow over the surface by the fluid being given intermittent ‘kicks’ to keep the fluid moving.

[0009] The pulsed signal may comprise a pulse envelope containing a varying signal which may, optionally, comprise a train of shorter duration pulses. Advantageously, the pulse envelope may contain 10 to 100 pulses. Conveniently, the pulse envelope duration and the pulse envelope repetition period may be independently adjustable.

[0010] Optionally, the plasma generator is operable to cause a change in direction of the flow of the fluid over the surface primarily in a single direction. To this end, the first electrode may comprise first and second elongate elements which may be in juxtaposed and, optionally, substantially parallel alignment. Advantageously, the first and second elongate elements extend generally parallel to the usual direction of motion of the surface in use. Optionally, the apparatus is operable such that first elongate element receives a first pulse and the second elongate element receives a second pulse after a time interval at least as long as the time taken for fluid to travel between the first and second elongate elements. Advantageously, the apparatus is operable to drive the first and second elongate elements with a common pulsed signal generated by the signal generator, the period of the pulsed signal being at least as long as the time taken for fluid to travel between the first and second elongate elements. In this way, the fluid may be repeatedly ‘kicked’ in a common direction.

[0011] Optionally, the plasma generator is operable to cause a change in direction of the flow of the fluid over the surface in alternate generally opposite directions. The plasma generator may further comprise a second electrode operable independently of the first electrode in response to a pulsed signal generated by the signal generator. Providing first and second independently controllable electrodes provides far greater flexibility of operation over the prior art system of providing a single set of a multiplicity of parallel conductive lines all electrically connected to one another. For example, the plasma generator may be operable to cause the fluid to flow in alternate generally opposite directions along the surface. In this way, spanwise oscillations may be created. If these generally opposite directions are generally perpendicular to the principal direction of fluid flow over the surface (caused, for example, by movement of the surface through the fluid), this may tend to reduce drag.

[0012] Conveniently, the first and second electrodes are in juxtaposed alignment and may, optionally, be in substantially parallel alignment. Advantageously, the first and second electrodes extend generally parallel to the usual direction of motion of the surface in use.

[0013] Optionally, the signal generator is operable to supply pulses to the first and second electrodes alternately thereby driving the first and second electrodes alternately. There are two currently preferred modes of operation of this type. In the first mode, pulses are used to drive first and second electrodes spaced apart over a time interval slightly longer than the time taken for fluid to travel between the first and second electrodes. In this way, the fluid is repeatedly kicked in a common direction, akin to the mode described above for a first electrode comprising first and second electrodes. In the second mode, pulses are used to drive first and second electrodes spaced apart over a time interval slightly shorter than the time taken for fluid to travel between the first and second electrodes. In this way, the fluid is kicked back in the opposite direction as it approaches the second electrode from the first electrode. If this is repeated, the fluid is repeatedly kicked from one electrode to the other and spanwise oscillations can be obtained.

[0014] Optionally, the plasma generator includes a dielectric that supports the first electrode and any second electrode on a first side thereof. Electrodes may be formed on surfaces of the dielectric layer or could be within the dielectric layer. Conveniently, the dielectric may be in the form of a flexible sheet so that it may be attached to curved surfaces such as an aircraft wing. Optionally, the dielectric comprises a second side that supports an opposed electrode of the plasma generator, the first and second sides being generally opposed.

[0015] The first and second electrodes may comprise a plurality of electrically connected elongate elements which may be arranged such that the elongate elements are interdigitated. Conveniently, the opposed electrode may comprise a plurality of electrically connected elongate elements. The elongate elements of the first, second and opposed electrodes may be in a substantially parallel juxtaposed alignment when viewed facing the first side of the dielectric. Advantageously, the elongate elements of the first, second and opposed electrodes may extend substantially parallel to the usual direction of motion of the surface. The elongate elements of the opposed electrode may be laterally offset from the elongate elements of the first and second electrodes.

[0016] The present invention also relates to an aircraft aerodynamic surface and an aircraft including apparatus as defined above, wherein the plasma generator is operable to cause a change in direction of the flow of the fluid over the surface.

[0017] According to a second aspect of the present invention, there is provided a method of influencing fluid flow over a surface, comprising the step of driving an electrode provided on the surface with a pulsed signal thereby to generate a plasma and, in turn, to cause a change in direction of the flow of the fluid over the surface.

[0018] For a better understanding of the present invention, embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:—

[0019]FIG. 1 shows an aircraft wing on which an electrode arrangement in accordance with the present invention is formed;

[0020]FIG. 2 is a schematic diagram of the apparatus for generating a plasma;

[0021]FIG. 3 shows the timing of the pulses applied to the electrodes;

[0022]FIG. 4 shows in more detail the waveform of the pulses applied to the electrodes; and

[0023]FIGS. 5A and B show an alternative embodiment of the plasma generating apparatus in perspective and cross-section, respectively.

[0024] In the drawings, like elements are generally designated with the same reference numerals.

[0025]FIG. 1 shows a wing model 1 to which an electrode assembly 3 in accordance with the present invention is attached. The leading edge of the wing 1 is designated 5.

[0026] The electrode assembly 3 comprises first and second electrodes 7 and 9.

[0027] The electrodes 7 and 9 are similar in shape, and have a generally comb-like structure. Each electrode 7 and 9 comprises a plurality of parallel, vertically extending (in FIG. 1) fingers which are connected by a horizontal (in FIG. 1) strip. The fingers and the strip of each electrode 7 and 9 are integrally formed with one another. The first electrode 7 has a terminal 11 for connection to a power supply and second electrode 9 has a terminal 13 for connection to a power supply.

[0028] In the drawings only a limited number of electrode “fingers” are shown, for the sake of clarity. It will be understood that many more fingers would be employed in an electrode assembly for a commercial aircraft.

[0029] The first 7 and second 9 electrodes are interdigitated. As shown in FIG. 2, the electrodes 7 and 9 are formed on a sheet 15 of dielectric material, such as a polyester sheet which, in the embodiment, is 250 μm thick. A third, planar sheet electrode 16 is formed on the opposite side of the dielectric layer 15 to the first and second electrodes 7 and 9. The first, second and third electrodes 7, 9 and 16 are formed from copper and are 17 μm thick. The first and second electrodes 7 and 9 are formed by a conventional etching process. The fingers of each of the first and second electrodes 7 and 9 are between 200 μm and 500 μm wide, with each finger being spaced apart from its adjacent finger by 4 mm (adjacent fingers will be of different electrodes).

[0030] The first, second and third electrodes 7, 9 and 16 are driven by alternating current high-tension power supply 18.

[0031] The electrode assembly 3, comprising the three electrodes 7, 9 and 16 and the dielectric layer 15, is formed as a flexible sheet. The electrode assembly 3 can be adhered to a surface where it is required, such as an aircraft wing or fuselage. The flexibility of the sheet allows the electrode assembly 3 to be attached to curved surfaces, and the electrode assembly 3 is retro-fittable to existing aircraft with minimal structural disruption.

[0032] If the aircraft wing, or the structure to which the electrode assembly 3 is attached, is of metal or other electrically conductive material, the third electrode 16 may not be formed, and the conductive structure may be used to provide the function of that electrode by connecting the conductive structure to the power supply 18.

[0033] The power supply 18 is configured to alternately drive at the desired span-wise oscillation frequency first electrode 7 and second electrode 9.

[0034]FIG. 3 shows the duration and timing of the electrical pulses applied to terminals 11 and 13 of the first 7 and second 9 electrodes respectively. The upper oscillation current pulses shown in the Figure are applied to first electrode 7 and the lower oscillation current pulses are applied to second electrode 9. As can be seen, a pulse is never applied to the first 7 and second 9 electrodes at the same time. Each pulse envelope 20 comprises a plurality of shorter duration plasma pulses 22.

[0035] The signals have a pulse envelope repetition period T, determining the span-wise oscillation frequency. The Jung paper “Suppression of Turbulence in Wall-bounded Flows by High Frequency Spanwise Oscillation” referred to above describes how to select a span-wise oscillation frequency range that will reduce drag. Periods of oscillation T⁺ _(osc) ranging from 25 to 200 were studied, where

T ⁺ _(osc) =T _(osc.) U _(τ) ¹/ν

[0036] T_(osc) being the oscillation period of the wall

[0037] U_(τ) being the wall friction velocity

[0038] ν being kinematic viscosity.

[0039] It was found that T⁺ _(osc)=100 produced the most effective suppression of turbulence.

[0040] Within each pulse envelope 20 of the signal is a train of high repetition-rate pulses 22, for example 10 to 100 pulses. FIG. 4 shows in more detail the voltage waveform 24 and the current waveform 26 of one pulse 22. The number of pulses 22 within each pulse envelope 20 and the energy of these pulses can be varied to allow adjustment of the impulse imparted to the air in the boundary layer. Generally, the greater the number and the energy of the pulses 20, the greater the effect on the boundary air. However, increasing the number of pulses, and the energy of each pulse will result in increased power consumption. Since, when implemented on an aircraft wing, the device is intended to allow a reduction in fuel consumption of the aircraft engines, there will be no overall saving in energy consumption if the power consumed by the plasma generating apparatus equals the energy saved by the reduction in drag. The energy applied to the pulse generating apparatus should be chosen to reduce the overall energy consumption of the aircraft in flight.

[0041] The duration D of the pulse envelope and the pulse envelope repetition period T can be independently adjusted.

[0042] Plasma is initiated by the high electric field at the first 7 and second 9 electrode/dielectric 15/air triple point 27, and then spreads out, capacitively coupled, to the third, planar electrode 16 on the opposite side of the dielectric layer 15 to the first and second electrodes 7 and 9. The electrode assembly 3 has an inherent capacitance, and additional capacitance when the plasma is formed.

[0043] The plasma generates span-wise impulses 28 in the boundary layer at the span-wise oscillation frequency. The impulses 28 are created by the plasma heating and causing expansion of the air in the boundary layer adjacent to the first 7 and second 9 electrodes. The impulses 28 move in a span-wise direction, which is generally perpendicular to the direction 30 of the primary airflow over the aircraft wing 1. It is considered by the present inventors that generating impulses generally perpendicular (within ±10°) to the principal direction 30 of airflow reduces drag.

[0044] Airflow is generated in a span-wise oscillating fashion because adjacent electrodes 7 and 9 are alternatively driven. When the first electrode 7 is driven, as mentioned above, the application of power to the electrode causes the heating and expansion of the air adjacent to the electrode 7. The expanding air will radiate from the electrodes, with components of the expanding air moving in opposite span-wise directions. During the period L when no pulses are applied to any of the electrodes the fluid will continue to flow. When the second electrode 9 is driven the air adjacent to this electrode 9 will also be heated and expand. This expansion will serve to reverse the span-wise movement of the air caused by the previous pulse supplied to the first electrode 7. The repeated alternate application of pulses to the electrode 7 and 9 therefore causes span-wise oscillation of air adjacent to electrodes 7 and 9 on the wing 1 surface. A suitable oscillation frequency range is thought to be between 10⁴ and 10⁵ Hz. The value will be chosen according to the location of the electrode assembly 3 on the aircraft and the speed of the aircraft.

[0045] In the embodiment the plasma spreads out approximately 4 mm on either side of each of the electrode fingers when a peak voltage of approximately 4 kV is applied.

[0046] The power supply 18 may generate a semiconductor switched current pulse which is already at a sufficiently high voltage for plasma generation, or, if not, the pulse is fired into a step-up transformer. For non-resonant charging of the electrode assembly 3 the output can be taken through a charging resistor. Resonant charging can also be used from a supply with no charging resistor. Current flows and when a plasma generation threshold is exceeded, plasma is generated thereby dissipating power. In the present embodiment, when a sheet 15 of 250 μm thick polyester is used for the dielectric material, the plasma generation threshold will be approximately 2 kV. Integrating the voltage and current waveforms shows the energy balance—this rises as the structure charges and then drops as it discharges but not by as much as it rose, the difference is the energy dissipated in the plasma. Multiplying by the plasma pulse repetition rate and the duty cycle gives the average power dissipation. Dividing by the electrode sheet surface area gives the power per unit area. This is an important factor—in order for drag reduction to be efficient, the power per unit area must be less than the power drag reduction of the skin friction. Pulses of both polarities are used for plasma generation.

[0047] In the embodiment the pulse rise time is short compared to the pulse period. Typical energy dissipations per centimetre of electrode length per pulse at 4 kV are approximately 30 pJ. There can be differences between positive and negative polarities but this is an approximate figure. It is estimated that air velocities of >1 ms⁻¹ can be generated within a few millimetres of the first and second electrodes 7 and 9 and close to the surface of the dielectric layer 15.

[0048]FIGS. 5A and 5B show an alternative arrangement where only a single electrode 32 is provided on the upper (in FIG. 5) surface of the dielectric layer 15. On the lower (in FIG. 5) surface of the dielectric layer 15 a further electrode 34 is formed. The upper electrode 32 is comb-like and is similar in configuration to either of the first 7 or second 9 electrodes of the first embodiment. The lower electrode 34 is of a similar configuration to the upper electrode 32 but is laterally offset with respect to the upper electrode 32, as shown particularly in the cross-sectional view of FIG. 5B. Because the electrodes 32 and 34 are offset, the plasma generated is generally confined to one side 36 of the electrode 32. Because the plasma is generated to only one side of the electrode 32 this will cause heating and expansion of the air generally in a single direction 38.

[0049] In this embodiment, if the aircraft wing, or the structure to which the electrode assembly 3 is attached, is of metal or other electrically conductive material, an insulator should be provided between the lower electrode 34 and conductive structure.

[0050] The electrode 32 is driven with a signal having a configuration of one of the waveforms shown in FIG. 3, i.e. a periodically repeating pulse envelope 20 containing a series of high frequency pulses 22. The timing of the pulse envelopes 20 is selected so that the distance D travelled by the air driven by a first pulse envelope is equal to or greater than the distance d between adjacent fingers of the electrode 32. Therefore, air moved as a result of plasma generation by first electrode finger 32.1 will have flowed in a span-wise direction to or past electrode finger 32.2 when the subsequent pulse is applied. The subsequent pulse will push this air further in the direction 38, thus continuing the flow in a single span-wise direction along the wing.

[0051] By timing the pulse envelope repetition period T so that it coincides with the air impulse transit time a coherent boundary layer travelling wave is generated which will disrupt turbulent flow structures.

[0052] Although in the first embodiment the air oscillates in opposite span-wise direction, and in the second embodiment the air travels in a single span-wise direction, in both the embodiments it will understood the movement of the air generated by the plasma generating apparatus is generally perpendicular to the direction 30 of the main airflow.

[0053] The plasma generating apparatus may be controlled so that it does not operate during take-off and landing of the aircraft, and generally at low altitudes. It may be advantageous not to operate the plasma generating apparatus in these situations for safety reasons—operation may interfere with electronic apparatus in the aircraft and also on the ground, at the airport. 

1. Apparatus for influencing fluid flow over a surface, the apparatus including a plasma generator comprising a first electrode and a signal generator, the apparatus being operable to drive the first electrode with a pulsed signal generated by the signal generator thereby to cause a change in direction of the flow of the fluid over the surface.
 2. Apparatus according to claim 1, wherein the pulsed signal comprises a pulse envelope containing a varying signal.
 3. Apparatus according to claim 2, wherein the pulse envelope contains a train of shorter duration pulses.
 4. Apparatus according to claim 3, wherein the pulse envelope contains 10 to 100 pulses.
 5. Apparatus according to claim 2, wherein the pulse envelope duration and the pulse envelope repetition period are independently adjustable.
 6. Apparatus according to claim 1, wherein the plasma generator is operable to cause a change in direction of the flow of the fluid over the surface primarily in a single direction.
 7. Apparatus according to claim 6, wherein the first electrode comprises first and second elongate elements.
 8. Apparatus according to claim 7, wherein the first and second elongate elements are in juxtaposed alignment.
 9. Apparatus according to claim 8, wherein the first and second elongate elements are in substantially parallel alignment.
 10. Apparatus according to claim 9, wherein the first and second elongate elements extend generally parallel to the usual direction of motion of the surface in use.
 11. Apparatus according to claim 7, wherein the apparatus is operable such that first elongate element receives a first pulse and the second elongate element receives a second pulse after a time interval at least as long as the time taken for fluid to travel between the first and second elongate elements.
 12. Apparatus according to claim 11, wherein the apparatus is operable to drive the first and second elongate elements with a common pulsed signal generated by the signal generator, the period of the pulsed signal being at least as long as the time taken for fluid to travel between the first and second elongate elements.
 13. Apparatus according to claim 1, wherein the plasma generator is operable to cause a change in direction of the flow of the fluid over the surface in alternate generally opposite directions.
 14. Apparatus according to claim 13, wherein the plasma generator further comprises a second electrode operable independently of the first electrode in response to a pulsed signal generated by the signal generator.
 15. Apparatus according to claim 14, wherein the first and second electrodes are in juxtaposed alignment.
 16. Apparatus according to claim 15, wherein the first and second electrodes are in substantially parallel alignment.
 17. Apparatus according to claim 16, wherein the first and second electrodes extend generally parallel to the usual direction of motion of the surface in use.
 18. Apparatus according to claim 13, wherein the signal generator is operable to supply pulses to the first and second electrodes alternately thereby driving the first and second electrodes alternately.
 19. Apparatus according to claim 18, wherein the apparatus is operable such that first electrode receives a first pulse and the second electrode receives a second pulse after a time interval less than the time taken for fluid to travel between the first and second electrodes.
 20. Apparatus according to claim 1, wherein the plasma generator includes a dielectric that supports the first electrode and any second electrode on a first side thereof.
 21. Apparatus according to claim 20, wherein the dielectric is in the form of a flexible sheet.
 22. Apparatus according to claim 20, wherein the dielectric comprises a second side that supports an opposed electrode of the plasma generator, the first and second sides being generally opposed.
 23. Apparatus according to claim 13, wherein each of the first and second electrodes comprise a plurality of electrically connected elongate elements.
 24. Apparatus according to claim 23, wherein the first and second electrodes are arranged such that the elongate elements are interdigitated.
 25. Apparatus according to claim 24, wherein the opposed electrode comprises a plurality of electrically connected elongate elements.
 26. Apparatus according to claim 25, wherein the elongate elements of the first, second and opposed electrodes are in a substantially parallel juxtaposed alignment when viewed facing the first side of the dielectric.
 27. Apparatus according to claim 26, wherein the elongate elements of the first, second and opposed electrodes extend substantially parallel to the usual direction of motion of the surface.
 28. Apparatus according to claim 26, wherein the elongate elements of the opposed electrode are laterally offset from the elongate elements of the first and second electrodes.
 29. An aircraft aerodynamic surface including apparatus according to claim 1, wherein the plasma generator is operable to cause a change in direction of the flow of the fluid over the surface.
 30. An aircraft including apparatus according to claim 1, wherein the plasma generator is operable to cause a change in direction of the flow of the fluid over the surface.
 31. A method of influencing fluid flow over a surface, comprising the step of driving an electrode provided on the surface with a pulsed signal thereby to generate a plasma and, in turn, to cause a change in direction of the flow of the fluid over the surface. 